The invention pertains to an apparatus for introducing and dispersing a gas in a liquid or slurry, preferably for aerating liquid-solid organic waste mixtures in order to accelerate the decomposition of the organic matter.
In the field of organic waste treatment, decomposition of organic matter, such as vegetable wastes and sewage, can be effected by introducing suitable bacteria into a tank containing a slurry of the organic matter in water, while aerating and mixing the slurry. Thorough aeration of the slurry, in the form of fine air bubbles dispersed throughout the slurry, accelerates such decomposition by providing ample oxygen to the bacteria.
In International Publication No. WO-97/11034, published Mar. 27, 1997, there is disclosed a submersible mixing impeller for use in effecting aeration of liquids in slurries. It has been found that in a reaction vessel, typically a cylindrical tank, having a diameter up to about 14 feet, a single such impeller is able to efficiently aerate and mix the slurry. However, in reaction vessels having larger diameters, a single mixing impeller is insufficient to circulate the entire volume of slurry, resulting in uneven digestion of the biodegradable material, as evidenced by uneven temperatures in different parts of the tank. As a result, longer periods of time are required to effect complete digestion of the waste material. The use of such larger reaction vessels is very desirable in the treatment of organic waste mixtures, in order to achieve economies of scale. There is accordingly a need for an apparatus for efficiently aerating large batches of liquid-solid organic waste mixtures.
The invention provides an apparatus which can both introduce sufficient volumes of air (or other gas) into a slurry (or liquid) and at the same time circulate the slurry in a manner that is constantly bringing non-aerated slurry into proximity with the aerators. This is achieved by providing a reaction vessel with a radially inner zone for the downward flow of unaerated slurry, a radially outer zone, having a plurality of aerators, for the upward flow of aerated slurry, and a mixing propeller at the bottom of the inner zone to draw slurry downward in that zone and disperse it into the outer zone.